Engine out cold-start emissions generated before light-off of an exhaust system catalytic converter may contribute a large percentage of the total exhaust emissions. Various approaches may be used by engine control systems to expedite the attainment of the catalyst light-off temperature. For example, various combinations of valve overlap and increased fuel injection may be used to expedite catalyst warming.
However, the inventors herein have recognized a potential issue with such approaches. Specifically, turbine spool-up may be delayed during the cold-start while the catalyst approaches a light-off temperature. Thus, boosted engine performance during the cold-start may be degraded. Furthermore, if the vehicle operator tips-in while the catalyst is warming up, there may not be sufficient turbine speed to meet the torque demand, resulting in driveability issues.
In one example, the above issues may be at least partly addressed by a boosted engine system that uses a binary flow turbine. In one example, boosted engine performance may be improved by a method comprising: in response to a cold-start condition, adjusting a scroll valve coupled to an inlet of one scroll of a multi-scroll exhaust turbine to expedite catalyst warm-up. Specifically, by closing the scroll valve during at least an early part of an engine cold-start, exhaust manifold pressure can be raised, expediting turbine spool-up and catalyst warm-up.
In one example, during an engine cold-start, an engine may be started with a scroll valve coupled to only an outer scroll of a multi-scroll exhaust turbine at a more closed position, such as at a fully closed position. The scroll valve may be maintained closed for a number of combustion events during the engine cold-start. By closing the scroll valve, exhaust flow to the turbine is restricted to a narrow passage, causing the exhaust to flow at a higher pressure to the turbine inlet. This expedites turbine spool-up and resulting turbine power. In addition, turbine surface area is reduced, increasing heat flux to the exhaust catalyst, and expediting catalyst light-off. Overall, turbo lag is reduced and driveability is improved at low engine speeds and boost levels.
As the exhaust temperature and turbine speed increase, the scroll valve may be opened. For example, once the exhaust catalyst has reached a light-off temperature and/or once the turbine speed is above a threshold speed, the scroll valve may be opened (e.g., fully opened). By opening the scroll valve, exhaust flow through the second scroll of the turbine is enabled so that the pressure in the exhaust manifold does not become excessive. In addition, turbine power and boost levels can be further increased. As such, this improves turbocharger control at higher engine speed and boost levels. It will be appreciated that wastegate and EGR valve adjustments may be coordinated with the scroll valve adjustment to further improve boost control during the engine cold-start.
In this way, scroll valve adjustments in a binary flow turbine can be advantageously used to improve turbine speed control and catalyst temperature control during an engine cold-start. By expediting catalyst light-off, cold-start emissions are reduced. By expediting turbine spin-up, turbo lag at low engine speeds is reduced, and boosted engine performance is improved. Overall, engine performance and exhaust emissions are improved.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.